Low-phase-noise single-frequency electrical sources are employed in numerous demanding applications including coherent communications, radar, radio astronomy, frequency metrology, and test and measurement. The use of a dual-frequency optical source in conjunction with electro-optical frequency division (EOFD) has enables construction of high-performance, ultra-low-phase-noise microwave-frequency sources. Such a state-of-the-art microwave source operating at 30 GHz has exhibited phase noise of −150 dBc/Hz at 10 kHz offset. That low level of phase noise is an improvement of about 30 dB relative to conventional microwave oscillators (e.g., based on up-converting from quartz or SAW oscillators). Electro-optical frequency division typically employs two lasers with high relative-frequency stability as the dual-frequency optical source, and electro-optical-modulator-generated optical frequency combs for the frequency division. The phase noise exhibited by the resulting microwave-frequency electrical signal is reduced by a factor of about N2 relative to the optical phase noise of the dual-frequency optical source, where N is the frequency division factor. However, any long-term frequency drift or fluctuations between the optical frequencies of the dual-frequency optical source are transferred to the microwave-frequency electrical signal, divided by N as a result of the electro-optical frequency division.
Examples of dual-frequency optical sources or electro-optic frequency division suitable for generating low-phase-noise microwave-frequency electrical signals are disclosed in:    [1] Li, J.; Yi, X.; Lee, H.; Diddams, S. A.; Vahala, K. J.; Electro-optical frequency division and stable microwave synthesis; Science 345(6194), 309-313 (2014);    [2] U.S. Pat. No. 9,450,673 entitled “Stabilized microwave-frequency source” issued Sep. 20, 2016 to Vahala et al;    [3] U.S. Pat. No. 9,537,571 entitled “Dual-frequency optical source” issued Jan. 3, 2017 to Li et al;    [4] U.S. Pub. No. 2016/0254646 entitled “Optical frequency divider based on an electro-optical-modulator frequency comb” published Sep. 1, 2016 in the names of Li et al;    [5] U.S. provisional App. No. 62/270,756 entitled “Dual SBS lasers from a non-reciprocal fiber cavity for stable microwave generation” filed Dec. 22, 2015 in the names of Li et al; and    [6] U.S. Pub. No. 2017/0302048 entitled “Stabilized non-reciprocal fiber-ring Brillouin laser source” published Oct. 19, 2017 in the names of Li et al.
Each of the preceding references in incorporated by reference as if fully set forth herein. In one or more of those references, the dual-frequency optical source comprises the dual stimulated Brillouin lasers (SBL) co-lasing from a high finesse optical resonator. The resonator typically is placed in a temperature-stabilized enclosure or housing. Temperature drift or fluctuations of the optical resonator will change its optical path length (due to both thermal expansion/contraction and refractive index change with temperature), thereby also changing the frequency difference between the dual Brillouin lasers as well as the resulting divided microwave frequency. In addition, some practical applications require that a given microwave-frequency source be phase locked or synchronized with an external clock signal with high long-term stability. It therefore would be desirable to develop methods for synchronizing the EOFD-based microwave-frequency source to an external reference or to improve the long term stability of the EOFD-based microwave-frequency source.